Methods and systems for distance measurement are used in many applications. Examples thereof include markedly precise measurements in geodetic applications, but also measurement tasks in the region of building installation or for industrial process controllers.
Stationary, movable or else handheld distance measurement devices are used for these tasks, and perform an optical distance measurement with respect to a selected measurement point. In this case, usually a laser beam is emitted and, after reflection at the target, is received again and evaluated. In this case, various measurement principles are available for determining the distance, such as e.g. phase or time-of-flight measurement.
Particularly in the field of building installation or acceptance of construction work, portable and handheld devices are used which are placed in relation to a structure to be measured and then carry out a distance measurement with respect to a surface. A typical handheld distance measurement device suitable for such applications is described for example in EP 0 738 899 and EP 0 701 702.
Since a measurement point that is visible on the surface to be measured is advantageous for most applications, red lasers are usually used as radiation sources for the distance measurement. In conjunction with great ease of handling, accuracies down to the millimeters range can be achieved with rangefinders in the prior art. Currently obtainable handheld distance measurement devices can carry out measurements from one point to another point to which there is a line of sight. If the target is concealed, horizontal mass can also be ascertained by means of an inclination sensor.
One possibility for determining a distance between two points, which can also be used if there is no line of sight between the points, is calculation by means of trigonometry. This is already known sufficiently from ground-based surveying devices, such as theodolites or total stations.
For trigonometrically ascertaining a distance a between two spatial points B and C, it suffices to know the distance to these two points from a third point A, and the angle α at point A between the sides b and c in the direction of the points B and C. The length of a can then be calculated by means of the cosine law:a=√{square root over (b2+c2−2·b·c·cos α)}
Although a conventional handheld distance measurement device from the prior art makes it possible to measure the distances b and c to the spatial points B and C exactly, a function for accurately and reliably determining the angle α is generally missing. Acceleration sensors that can be used expediently nowadays in handheld distance measurement devices, in particular with regard to price and size, cannot yield a sufficiently reliable value for α for distance calculation purposes, and compasses are susceptible to disturbance particularly in interiors of buildings; at best angles in the vertical can be ascertained with sufficient accuracy and reliability by means of inclination sensors.
The prior art describes various solutions with handheld distance measurement devices comprising laser rangefinders by means of which two points can be targeted simultaneously, wherein an angle between the emission directions of the two lasers can be determined.
Both of the documents DE 10 2007 043 496 A1 and JP 2008 116 249 A in each case disclose a handheld distance measurement device comprising two laser rangefinders that are rotatable relative to one another, wherein the angle between said rangefinders can be determined.
DE 102 14 742 A1, by contrast, describes a solution with two handheld distance measurement devices which are connected to one another pivotably, wherein the mechanical connection between the two distance measurement devices has means for detecting the angle.